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We have observed our own Galaxy, the Milky Way, in such exquisite detail that today we know its main visible components very well, namely the stars (hundreds of billions of them) and the interstellar gas and dust. With the help of time-consuming computer simulations, we can build and analyse virtual galaxies that look rather similar to the Milky Way. However, many models and scenarios exist, and we have still only a rough idea of how our Galaxy was formed, about 10 billions years ago, and how it evolved since.

The Andromeda Galaxy

The Andromeda Galaxy (Messier 31) is a giant spiral galaxy, slightly more than 3 millions light years away, which resembles our own Galaxy, the Milky Way.

The observation of galaxies when they have just formed and are still young is mandatory to choose between the different models, and to improve our understanding of these galaxy formation and evolution processes. This requires observing very distant galaxies, for which the light has taken billions of years to travel and reach our telescopes: looking far away means looking at how objects were a long time ago. But observing galaxies when the Universe was only a few billion years old is a big challenge. At such distances, galaxies look tiny and are dramatically faint.

The VLT at Paranal (Chile)

Giant telescopes like the VLT with its 8 meter diameter mirrors have a collecting power around four times larger than the previous generation of telescopes. We still need, however, to optimise the way we collect the few remaining photons received from these young galaxies, most of them being lost when they finally pass through the Earth atmosphere. The ESO second generation VLT instrument MUSE is built to face this challenge. It is an integral-field spectrograph based on a number of innovative technological developments. But before describing the instrument itself, let us examine what it actually needs to achieve.

Young galaxies are made up of young stars. Observations of typical young stars in our Galaxy show that these are very bright objects. This brightness, however, varies quite significantly from one colour (wavelength) to the next. Their emitted light is in fact mostly gathered in a few specific emission lines: most of these lines are associated with the Hydrogen atom, the most abundant element in the Universe and the major constituent of stars. We therefore expect the light from young galaxies to be mainly coming from Hydrogen emission lines. Rather than using an imaging device, which would collect the light from a rather wide spectral range (mixing many colours together), we can gain in sensitivity by trying to focus only on these particular emission lines. Such a winning strategy provides a gain that is sufficient to allow the detection of the targeted faint galaxies.

Hubble Ultra Deep Field (HUDF)

The Hubble Ultra Deep Field is today the deepest image of the Universe. It contains galaxies that are older than 10 billions years.

One classical way to perform this (colour) selection would be to use an imaging device with a narrow band filter centred on the wavelength of the expected Hydrogen emission lines. Unfortunately this would be rather inefficient because the light we receive fromthese young galaxies is red-shifted proportionally to their relative speed (the so-called Doppler-Fizeau effect). The shift depends on the distance of the objects we are looking at (and on the expansion speed of the Universe), which we do not precisely know. We are therefore in a very difficult situation, unable to define the optimum narrow band filter, or in other words which colour to select, to better detect the galaxies.

A better option would be to use a spectrograph. A spectrograph disperses the light (separates the different colours, or wavelengths), and it is then easier to identify the ionised Hydrogen lines even if we do not know their exact red-shift. When using a conventional spectrograph, a small slit aperture is used, and one needs to precisely position it on the object to be observed. When searching for galaxies, however, this is not feasible since we do not know beforehand where these galaxies are located on the sky.

Atomic Hydrogen Spectrum

We can now appreciate the obstacles facing astronomers: how can we detect young galaxies if we do not know where exactly they are located on the sky, and at which wavelength their redshifted Hydrogen emission lines can be found? The answer lies in the concept of integral-field spectroscopy or 3D spectrography: this is neither simply imaging, nor spectrography, but both at the same time. This concept was first pioneered at the Centre de Recherche Astrophysique de Lyon, France, and makes it possible to cover a complete field of view with spectra sampling it homogeneously. Such spectrographs can really be used as efficient explorers of the Universe in three dimensions (two for the sky positions, and one for the wavelengths).

The MUSE instrument is based on this very concept (more on Technology page). In order to be able to probe a field of view as large as possible, MUSE uses not just one, …, but twenty-four integral-field spectrographs! High optical efficiency is maintained by making use of a new technology: the advanced slicers. As mentioned above, the galaxies we are looking for appear tiny when observed at such distances. MUSE will use an innovative adaptive optics system which will correct for the perturbations due to the earth atmosphere in real time. The performance of MUSE should thus allow the detection of galaxies which appear a hundred million times fainter than the faintest stars observable with the naked eye. Even then, obtaining useful information on these remote objects with MUSE will take up to hundreds of hours with one of the 8 meter VLT telescopes.

Representation 3D de l’instrument MUSE

This unique and ambitious project is supported by seven major European research institutes: the Centre de Recherche Astrophysique de Lyon (France) which is the leading institute, the European Southern Observatory (ESO), the Leiden Observatory (Netherlands), the Laboratoire d’Astrophysique de Tarbes-Toulouse (France), the Göttingen Astrophysics Institute (Germany), the Astrophysics department of the Zurich Polytechnic Institute of Technology (Switzerland) and the Potsdam Astrophysikalisches Institut (Germany).

The MUSE consortium brings together more than a hundred scientists and engineers who cover all the required expertise to build and exploit this unique instrument: optics, mechanics, electronics, cryogenics, signal processing, management, observational and theoretical astrophysics. More details can be found in the Consortium and Teams sections.

Started in 2004, the instrument shall see its first light in 2012 at the VLT site (Paranal) in Chile. This will be the launch date for a restless hunt for young and distant galaxies, the goal being to attack the fundamental questions of the early stages of galaxy formation and evolution. But MUSE will also be an efficient tool to tackle astrophysical problems in many other fields. For instance, the MUSE consortium plans to study the environment of super-massive black holes, which are assumed to be present in most galactic nuclei, as well as to examine in great detail the stellar populations of nearby galaxies (see more detail and other examples in the Science section). The MUSE spectrograph will of course be opened to the ESO scientific community at large, which will certainly use its exceptional performance to advance our understanding of many different types of astrophysical objects observed in the Universe.

Computer simulation of a 3D deep field MUSE exposure.

Galaxies are coloured according to their distance (the redder the farest).